The present invention relates to a device for holding the braking force of a motor vehicle to allow the driver to start the motor vehicle on an uphill road easily.
When a motor vehicle stopped on an uphill road starts to move uphill, the driver has to coordinate the braking mechanism, accelerator pedal, and clutch of the motor vehicle to start the motor vehicle, without allowing the motor vehicle to move back and without stopping the engine.
Since such a starting process requires the driver to be highly skilled, there is a demand in the market for a motor vehicle which can easily be started on an uphill road.
To meet such a demand, the applicant has proposed a braking force retaining control apparatus as disclosed in Japanese Laid-Open Patent Publication No. 60-12360.
The disclosed braking force retaining apparatus is used on a motor vehicle having an automatic transmission with a friction clutch, which is controlled by an electronic control unit. The braking force retaining apparatus receives signals from various detectors for detecting operating conditions of the vehicle to retain a braking force when the vehicle is stopped by operating the brake. When the motor vehicle is to be started, the braking force is retained or held until the clutch is engaged by a clutch actuator. By retaining the braking force, the motor vehicle is allowed to restart on an uphill road smoothly without requiring skill on the part of the driver.
In the disclosed braking force retaining apparatus, when the motor vehicle is to be started, the operating condition of the friction clutch, i.e., a partly engaged position of the clutch is detected by a signal indicative of the amount of engagement of the clutch. When the torque of the engine is transmitted to the wheels through the clutch and a drive mechanism, a braking force holding valve which is a braking force holding mechanism disposed in a brake operating fluid circuit is opened to release the braking force. However, the above proposal cannot be incorporated in motor vehicles with manually operated transmissions since such motor vehicles have no means for detecting the amount of engagement of the clutch and hence a partly engaged clutch position cannot be detected. Another problem is that it is costly to equip a motor vehicle with means for detecting the amount of engagement of the clutch.